In one of the classes of optical information storage medium, a substrate is fabricated which has grooves formed in at least one surface of the substrate. Using a spin-coating technique, a dye material layer is applied to the surface of the storage media into which the grooves have been fabricated. The grooves can be used to define the track upon which information in the dye layer can be stored and accessed. The grooves are identified by a difference in phase for radiation traversing an optical path in the storage medium that includes a groove as compared to the phase of radiation traversing a path near to, but not including, the groove. This detection of this difference in phase for radiation transmitted by the storage medium, either using density tracking or phase tracking techniques, results in control signals which activate the servo tracking mechanisms. Through interaction with radiation applied to the storage medium, the optical properties of the dye material layer can be altered along the track defined by the groove(s). In this manner, information can be stored on the storage medium and, because of the alteration of the optical properties, the information can be retrieved through interaction of the storage medium with a suitable radiation beam and radiation detection apparatus.
Referring to FIG. 1, the cross-sectional structure of an optical information storage media suitable for advantageously using the present invention is shown. A substrate 11, typically comprised of a polycarbonate material, has a groove 11A fabricated therein, the groove having a depth of G and a width W. A dye material layer 12 of thickness L is deposited on the substrate 11, typically by a spin coating procedure. A reflecting coating 13 is then deposited on the dye layer. In the immediate vicinity of the groove 11A, the application of the dye material layer does not result in a flat surface, but has a depression of height cG, where c is referred to as the conformality, caused by the presence of the groove.
The difference in phase of a light beam reflected from a land (non-groove) area and a light beam reflected from groove area, when not a constant over the storage region of the storage medium, can result in a degradation of the ability of the detection system to differentiate between the two types of regions. As result degradation, the groove tracking can become inoperative. In European Patent Application Number 92109463.7, filed in the name of T. Mizukuki, T. Koike, N. Kitagawa, and S. Hirose, an attempt has been made to solve the problem of the radial dependence of the difference in phase as a function of radius by varying the depth G and/or the width W of the grooves as a function of radius. While this attempt to eliminate the radial dependence of the difference in phase has resulted in improvement of the tracking characteristics of the detection system, the difficulty in distinguishing between an optical path that passes through a land region from an optical path that passes through a groove has continued to limit the performance of an optical storage system. In addition, it has been determined that unless the variation in the depth of the groove(s) are small, changes in the amplitude of the (read-out) signals during the accessing of the information written in the dye material layer can degrade the performance of the information storage system.
A need has therefore been felt for an optical storage medium which provides a difference in the phase of a radiation beam transmitted through a groove and a radiation beam transmitted through a land area which is approximately constant as function of distance from the center of the storage medium. In addition, a need has been felt for a groove structure that can provide a the constant difference in phase over the information bearing portion of the storage medium without compromising the performance of the read-out of information stored in the dye material layer.